[LakeEffect]

Have a John Deere 180 lawn tractor. Few months ago replaced the plug, oil, oil filter, air filter and fuel filter. Did this after it had been running poorly for a little while (hesitating every few seconds while grass cutting). Seemed to run good after this maintenance, but then started feeling poor again.

After having it sit for 6 weeks or so, I took it out and put the snowthrower attachment on. Ran real rough, but I put it away. Now, just last night, I took it out to test. Tried to start it and it cranked away but wouldn't start. Waited a bit and tried again but it wouldn't even turn over (it's had a starting problem for years so this was totally unusual). I gave up and boosted it. It turned over right away while the choke was on and started then I took the choke off, but then there was some good smoke coming out of the exhaust and it stalled and backfired after a few seconds. I tried again and the same thing. Tried a third time and when it backfired there was definitely flames coming out the exhaust. Needless to say I stopped there!

Need to track down the problem. Where should I start, keeping in mind I'm only an average do-it-myselfer?